The average temperature of the planet for the next several thousand years will be determined this century—by those of us living today, according to a new National Research Council report which lays out the impact of every degree of warming on outcomes ranging from sea-level rise to reduced crop yields.

"Because carbon dioxide is so long-lived in the atmosphere, it could effectively lock Earth and future generations into warming not just for decades and centuries, but literally for thousands of years," atmospheric scientist Susan Solomon of the National Oceanic and Atmospheric Administration, who chaired the report, said at a July 16 press briefing held to release it. She compared CO2 to cheesecake: "If I knew that every pound of cheesecake that I ate would give me a pound that could never be lost, I think I would eat a lot less cheesecake."

According to the report, for every degree Celsius of warming, impacts include:

* A 5 to 15 percent lower yield for some crops, including corn in Africa and the U.S., and wheat in India
* A 3 to 10 percent increase in heavy rainfall globally
* A 5 to 10 percent drop in rainfall in southwestern North America, southern Africa and the Mediterranean, among other precipitation changes
* A 5 to 10 percent change (increases in some regions, decreases in others) in stream flow in many river basins globally
* A 15 to 25 percent decrease in the extent of Arctic Ocean sea ice

The report's authors were charged with evaluating a range of "greenhouse gas–stabilization targets and describe the types and scale of impacts likely associated" without any judgment on whether such targets are "technically feasible" or which is "most appropriate." In essence, the scientists evaluated the impacts associated with a given final level of carbon dioxide in the atmosphere, but did so through the lens of temperature change.

This represents a shift in the usual analysis of climate change, particularly in international negotiations, which tend to focus on how much concentrations of greenhouse gases in the atmosphere will rise by a particular date. "Many impacts respond directly to changes in global temperature, regardless of the sensitivity of the planet to human emissions of CO2 and other greenhouse gases," says geoscientist Katharine Hayhoe of Texas Tech University in Lubbock, a co-author of the report, excluding effects such as ocean acidification and CO2 as a fertilizer for plants. "Those impacts don't 'care' about what the CO2 concentration is."

It also eliminates much of the uncertainty surrounding potentially ill effects; whereas various mathematical models may disagree about when and at what concentrations Arctic Ocean sea ice disappears, they all agree that at roughly 3 degrees C of warming, the far north will be ice-free. "It's amazing how consistent they become," Solomon says. "At what point do you get to three to four degrees of warming, which is roughly the time when Arctic sea ice is mostly gone."

Adds economist Gary Yohe of Wesleyan University, another co-author: "We will commit to an ice-free Arctic sometime this century. We won't know definitively until 2090, but essentially there's nothing we can do about it at that point in time and it changes the climate system dramatically."

Already, the planet's average temperature has warmed by 0.7 degree C, which is "very likely" (greater than 90 percent certain) to be a result of the rising concentrations of greenhouse gases in the atmosphere, according to the U.N. Intergovernmental Panel on Climate Change. That's about half what can ultimately be expected from the roughly 390 parts per million of CO2 already in the atmosphere—the highest level the planet has experienced in at least 800,000 years.

CO2 in charge
One result of the survey of existing research undertaken by the scientists is the clear and dominant role played by CO2. Although a wide variety of greenhouse gases contribute to human-caused global warming, it is CO2, largely alone, that will determine the long-term climate, Solomon says. "If you reduce emissions of methane or black carbon, it would help you trim the peak warming that will be achieved in the next century or so," Solomon says. But it's the "cumulative carbon that will determine the long-term human footprint on this planet."

That's because it can take thousands of years to remove CO2 from the atmosphere without human intervention. So what matters most as far as total warming is the ultimate stabilized level of CO2. "It doesn't matter what road you pick to get there," Solomon notes. And achieving any stabilization target—whether 2 degrees C of warming or 450 ppm or 1,000 gigatons of carbon added to the atmosphere by human activity—will require at least an 80 percent cut in emissions from peak levels by the end of this century and, ultimately, zero emissions over the long term. "You can get there by cutting now at rates of 1 percent per year for the rest of the century or let carbon emissions rates grow for awhile and cut harder later to the tune of 4 percent per year," Solomon explains.

Yohe estimates the cost of achieving a more modest goal of holding warming to roughly 2 degrees C at a cost of 0.5 to 1.5 percent of gross domestic product for the U.S. by 2050, thanks to the expense incurred by, for example, replacing existing coal-fired power plants with renewables or retrofitting them with carbon-capture technology. That hardly impacts the U.S. economy at all. "With usual growth, we'd get to the same level of GDP in 2051 that we would have gotten in 2050," he says. "It's not an awful disaster. The hyperbole of 'all these green jobs' or 'we're going to trash the economy'—neither one [is] true."

Already, cities such as New York have adopted a risk-management approach to potential climate impacts—preparing for the prospects posed by already guaranteed global warming. By analyzing current building codes and the like, the New York City Panel on Climate Change determined the acceptable level of risk for its residents and is now prioritizing projects that hold to those same levels the perils from climate change impacts directly on the city, such as sea-level rise or more frequent heat waves. "You can't actually climate-proof a city," says Adam Freed, acting director of the city's Office of Long-Term Planning and Sustainability. But "the benefits of the things that make sense to do today greatly increase as our climate changes."

It remains to be seen whether there is economic value to the idea of overshooting a given target and then coming back down, given that the time frame of emission cuts matters less than the cumulative emissions of CO2, Yohe notes. It is clear that there is no environmental benefit to delay. "Climate change is already altering the character of the places we know and love," Hayhoe says. "Unchecked, it has the potential to impact nearly every aspect of human infrastructure and our natural environment—from our cities and roads to our forests and fields."

Regardless, the report notes that the planet has entered a new era, dubbed the Anthropocene, "during which the evolution of the planet's environment will be largely controlled by the effects of human activities, notably emissions of carbon dioxide." Hayhoe, for one, compares this report with a doctor's visit for Earth—the chronic disease being human-emitted carbon dioxide. "Many of us have had the experience of going to the doctor and receiving advice on how to improve our health by making wise lifestyle choices," she notes. "It's up to us to decide how much we are willing to change."

The catch is: the decision is not just for the planet today and its present generations, it is also for the planet and generations to come. "The impacts we may be experiencing now and in the next few decades before choosing to stabilize CO2 levels," Solomon notes, "would only be about half the eventual impacts."

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